The invention relates to a light source for a digital image projection system, comprising a thin-film resonant microchamber with an active phosphorus area arranged between a front and a rear film reflector and an electron source for generation of an image-like structure for stimulation of the phosphorus.
Twisted-nematic translucent liquid-crystal light valves with polysilicone electrodes (TN-LCLVs) currently dominate the market for digital projectors. Other basic techniques entail light-reflecting polymer dispersing liquid-crystal light valves (PD-LCLVs), reflecting nematic and other LCLVs. Digital mirror devices (DMDs) and grid-light valves (GLVs) belong to a type of light valves called mechanical-electrical Microsystems (MEMS). As shown in FIG. 2, this type of spatial-light-modulating (SLM) display technology contains a light source 10 having a power supply 12; and a lamp 14 converting the electrical current of the power supply into an unmodulated light beam 16.
The light beam 16 is modulated by an image-generating unit 18 which containsxe2x80x94for examplexe2x80x94a two-dimensional spatial light modulator 20 supplied with image information from a digital image source 22. The spatial light-modulated light is then projected onto a screen 24 in an image projection unit 28. What all these SLM techniques have in common is that they passively modulate an incoming light beam in 1 or 2 dimensions for generating an image. This type of image projection unit needs a high-performance and small light source with a highly optimized illumination system in order to divert or attenuate enough light in the intake pupil of a projection lens.
As shown in FIG. 3, an alternative method of spatially modulating light for image projection is to generate active emission elements which can be projected through a lens with appropriate aperture. In digital and video projection systems of this type the image generation 18 is achieved by a special cathode-ray tube (CRT) which has already been developed and is well established in the market for high-performance applications. These special cathode-ray tubes usually comprise an evacuated glass tube containing a single cathode electron source and an array of elements for generation and control of the electron beam 30, an acceleration section 32 and a phosphorus powder anode 34. The image generated on the phosphorus anode 34 is projected onto a screen 24 by a projection lens (not shown). Some brightness limitations inherent to the system exist on account of the material properties of the powder phosphorus used in the anode 34. It is for example not easy to collect the Lambert emissions through a projection lens, since the maximum energy density load on the powder anode is limited by the thermal conduction. The individual cathode source 30 also limits the beam current and hence the maximum energy density load on the anode 34.
U.S. Pat. No. 5,469,018 discloses a resonant microchamber display element of high efficiency and high-aligned output. In a design suitable for projection screen television, the resonant microchamber display element is contained in the face plate of a cathode-ray tube (CRT). In this way the anode 34 in FIG. 3 is replaced by a resonant microchamber anode. There are a number of drawbacks in the use of RMCs in a CRT. Cathode luminescence through a cathode-ray tube, although representing a conventional method for image generation, does not lead to a flat, thin display device. Conventional projection CRT cathodes are limited in their output (current load) and service life. For applications with high light output, the image control electronics must also be configured for high performance and is more expensive.
U.S. Pat. No. 5,543,862 discloses a video display system for use in applications for direct viewing. The system has a flat panel image amplifier onto which a video image is transmitted, for example from an electro-luminescent panel in contact with the image amplifier panel. The image amplifier contains a layer of photocathode material for converting photons from the optical display into electrons, which are accelerated in a flat vacuum chamber and impact on a fluorescent layer, where they generate a light image. A crucial drawback of this arrangement for use in a projection display is that the light emission from the image amplifier is heavily diffused, entailing the use of larger optical focusing equipment. A further drawback is the relatively low efficiency of the system.
The object underlying the present invention is therefore to provide an improved image-generating light source and a projection display system that avoids the drawbacks of the prior art.
In accordance with a design example, the invention relates to a digital image projection system with a resonant microchamber display device comprising a plurality of image-generating light sources. The light images generated from the plurality of light sources are combined and projected onto a projection screen. Each light source comprises: a primary electro-luminescent image-generating unit, comprising a layer of electro-luminescent phosphorus and an array of control electrodes for generation of a luminescent image in the phosphorus layer; an image amplifier with a photocathode adjacent to the electro-luminescent phosphorus layer and generating electrons as a result of light from the primary electro-luminescent image, in order to emit a light image of high intensity, with the light image having a band width in the magnitude of 5 nm and an angle emission profile in the magnitude of 20xc2x0.
The digital image projection system in accordance with the invention has the advantages of a higher system efficiency thanks to lower emission angles and low power input of the image-generating light source. The system in accordance with the invention has the capability of generating a better image quality and higher contrast than conventional projection systems, and the image definition is not dependent on the brightness, as is the case in conventional CRT projection systems. The system also has the capability to generate a higher color saturation thanks to very narrow spectral emission of the red, green and blue color channels, it can be constructed smaller and lighter, and it generates less noise than conventional systems thanks to the use of smaller cooling systems and lower current.
The image-generating light source used in the projection system in accordance with the invention has the advantage that the photocathode is a dependable and long-lived electron source for the resonant microchamber anode. With close alignment the photocathode is a flat and thin electron source for the resonant microchamber anode. Since the image information is inserted early in the image-generating process (before the electro-optical amplification), the image control electronics can manage with lower current (lower control voltage and current) and can therefore be less expensive. The use of a photocathode in the image-generating light source permits the use of a non-pixellated monochromatic anode, for which reason an intermediate step for alignment is not necessary. The color image is generated by the combination of images generated by red, green and blue emitting light sources.